Many types of semiconductor devices have been investigated in recent years in attempts to obtain, for example, devices with desirable characteristics such as enhanced carrier mobility. For example, U.S. Pat. No. 4,163,237 issued on July 31, 1979 to Raymond Dingle, Arthur C. Gossard, and Horst L. Stormer, describes a multilayered device having a plurality of alternate wide and narrow bandgap layers with modulated doping, i.e., the wide bandgap layers are more heavily doped than the narrow bandgap layers, and high mobility. The modulated doping permits the device, commonly termed a quantum well structure, to have electron mobilities which are higher than those in devices having uniformly doped films. See Applied Physics Letters, 33, pp. 665-667, Oct. 1, 1978. In one embodiment of such devices, the devices are fabricated with the AlGaAs/GaAs materials system.
Such quantum well structures, which, of course, need not have modulated doping, have been extensively investigated. For example, multiple layer AlGaAs/GaAs quantum well structures that were grown by molecular beam epitaxy (MBE) have been shown to possess atomically smooth interfaces as well as uniform layer thicknesses. See, for example, Applied Physics Letters, 29, pp. 323-325, Sept. 15, 1976. Photoluminescence spectra from these structures were predominantly intrinsic. See, for example, Applied Physics Letters, 38, pp. 965-967, June 15, 1981. This fact suggests both high sample purity and luminescence efficiency.
However, several aspects regarding operation and characteristics of these devices have remained puzzling to workers in the field. For example, luminescence studies of some structures having a single GaAs quantum well showed extrinsic, impurity dominated luminescence as well as nonuniformities in the layer thickness. Further, inequivalences were observed in the electrical transport properties of AlGaAs/GaAs single interfaces which apparently depended upon which layer was grown first. Additionally, in modulation doped single interfaces having the GaAs layer grown first, electron mobilities were observed which were higher than those observed in multi-quantum well structures. See, Applied Physics Letters, 39, pp. 912-914, Dec. 1, 1981. In structures having reverse single interfaces in which the AlGaAs layer was grown first, however, the electron mobility was either lower or no mobility enhancement was observed. It should be further noted that it has been reported that multiple quantum well structures, in which each quantum well contains a reverse interface, show more mobility enhancement than the single reverse interface structures. See Journal of Applied Physics, 52, pp. 1380-1386, March 1981.